Buoyant media flotation

ABSTRACT

A clarification system for fluids in which particles are separated from the fluid by flotation and in which the buoyant media inducing the flotation is recycled. Flotation can be assisted by a flotation assistance device, such as dissolved air flotation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 09/506,780filed Feb. 18, 2000, now U.S. Pat. No. 6,890,431.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to treatment systems for clarification ofa fluid stream and, more particularly, to buoyant media introduced intothe flow to induce a rising force on particles to be removed from thefluid.

2. Background of the Invention

Technologies for the separation of components of a fluid stream on thebasis of density are well known in the art. The simplest systems rely ondifferences in densities alone to accomplish this separation, withheavier components settling, in the absence of agitation, to the bottomof the fluid for removal.

For small particles, the time required for the unassisted settlingprocess can be unacceptably long. Stokes' Law predicts that sphericalparticles suspended in a fluid medium settle at a rate proportional tothe second power of the particle diameter. Thus, large particles willsettle much faster than smaller ones. To increase the rate of settling,coagulants can be used. A coagulant is a substance that produces aloosely-massed precipitate. As the precipitate forms, it entraps solidparticles contained in the fluid. Aluminum sulfate and ferric sulfateare among the materials used in the art as coagulants.

Particles in solution often have a net surface charge. As a result, theytend to repel each other and resist the formation of agglomerates.Coagulation is the process whereby these repulsive charges areneutralized. After charge neutralization, the particles begin to collectinto larger aggregates. This agglomeration process is known asflocculation; the resulting aggregate is known as a floc. Flocculantaids are thus effective as solid-liquid separating agents. Flocculantaids are typically synthetic water soluble polymers based on acrylamide.There are some natural occurring polymers, such as chitosan, that can beused for this purpose.

Entrainment can be used in conjunction with flocculation to expedite theseparation process. For example, grains of sand can be introduced to thefluid to be treated. The floc that forms entraps the grains of sand, andthe density of the floc is increased as is its settling speed.Alternatively, bubbles may be generated as the floc is formed, and theirentrainment produces a floc that rises to the surface of the fluidrather than settling to the bottom. However, additional equipment andoperating expenses are required to generate the bubbles. Other settlingmedia may be selected on the basis of specific gravity, particle size,surface properties, or chemical properties. Increasing the concentrationof these settling media improves the efficiency of the flocculationprocess, because more particle collisions result and the size of theresulting agglomerated particles increases more rapidly. Also, thesettling capacity of the system increases with concentration. However,increasing the concentration of entrained materials increases the costof the separation process, and increases the volume of the wastematerials produced. For settling media with specialized properties,these cost constraints can be significant.

Solids separation processes employing settling inherently require energyin the flocculation stage, produce large volumes of sludge, produceeffluent with small, difficult to settle particles, and require large,complex process vessels. These characteristics lead to increased costs.Accordingly, a need exists to reduce the costs associated with solidsseparation in high rate fluid clarification systems.

SUMMARY OF THE INVENTION

In order to meet this need, the present invention is a clarificationsystem for fluids, in which particles are separated from the fluid byflotation and in which a buoyant media inducing the flotation isrecycled. Buoyant media separation processes according to the inventionoffer a number of advantages over settling media processes according tothe invention. Less energy is required in the flocculation stage in abuoyant media separation process. The volume of sludge produced islower, and the sludge solids concentration is higher; no further sludgeconcentration is required. Also, process vessel depth is substantiallyless for buoyant media separation processes, and process vesselconstruction is simpler and more economical.

Fluid to be treated passes into a mixing chamber in which it is mixedrapidly with coagulants and flocculants. The resulting mixture thenpasses into a flocculation chamber in which the buoyant media are addedand mixed and in which the flocculation process produces agglomeratedparticles. The product of the flocculation chamber passes into aflotation chamber. The flotation chamber is configured so that theremoval of clarified fluid draws the fluid entering the chamber upward,hastening the generation of a sludge layer containing separatedparticles and buoyant media at the top of the chamber.

Optionally, the flotation chamber can be equipped with a flotationassistance device, such as dissolved air flotation, diffused air, orfroth flotation system, to further hasten the flow of separatedparticles and buoyant media upward. In addition, lamella plates, tubesor other coalescing surfaces can be added to the flotation chamber toimprove the separation of the buoyant flocs. Submerged membrane systemscan be provided in the separation tank to remove the need for downstreamfiltration systems.

The clarified fluid obtained from the separation of sludge is removedfrom the bottom of the flotation chamber, and is optionally filtered.The sludge layer is removed from the top of the flotation chamber by anyof a number of methods of removal well known in the art, such as amechanical skimmer or by hydraulic desludging. The sludge layer thenpasses into a buoyant media recovery unit, wherein the buoyant media areseparated from the sludge stream. The sludge stream is disposed of andthe buoyant media are recycled into the flocculation chamber. Thebuoyant media recovery unit can make use of various types of devices toeffect the separation, such as a centrifugal separator, centrifugalscreener, cross-flow sieve, or vibratory screen separator. Alternately,the flocs can be subjected to shearing forces to separate the media fromthe waste particles, and the two phases are then allowed to separate.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram of components and the pattern of flow ofthe clarifier device of the present invention.

FIG. 2 is a schematic diagram of a flotation chamber of the presentinvention, including lamella plates.

FIG. 3 is a schematic diagram of a flotation chamber of the presentinvention, including tubes.

FIG. 4 is a schematic diagram of a flotation chamber of the presentinvention, including an immersed membrane module.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The high rate clarification system of this invention makes use of highconcentrations of particles of a buoyant media. The high concentrationsimprove the efficiency of flocculation and enable the system to producea high rate of flotation.

Referring to FIG. 1, fluid to be treated flows into a mixing chamber 10.A coagulant is added to the fluid in this chamber 10. The coagulants aretypically metal salts, such as aluminum sulfate, ferric sulfate, orPAC1, or can be any of the coagulant-type polymers currently offered bychemical suppliers. The amount of coagulant needed depends on factors,such as the concentration of solids to be removed from the fluid, but istypically in the range of 5 and 180 milligrams per liter of fluid.

After coagulant is added, the resulting mixture passes through aflocculation chamber feed line 14 into a flocculation chamber 20. In theflocculation process, aggregate suspended particles grow in size as theycombine. Buoyant media and a flocculant aid are added to the fluidmixture in chamber 20. A flocculant is added to chamber 20 with thebuoyant media. Typical flocculants for this application are syntheticwater soluble polymers based on acrylamide. There are also somenaturally occurring polymers that can be used such as chitosan. Theamount of flocculant needed depends on factors, such as theconcentration of solids to be removed from the fluid, but is typicallyin the range of 0.1 to 1.0 milligrams per liter of fluid. In theflocculation process, aggregate suspended particles grow in size as theycombine. The buoyant media may be finely divided solids, but may also besolids that have been foamed or otherwise provided with internal voids.The buoyant media may include a plastic powder which has a specificgravity less than 1.0. The plastic can be, for example, polypropylene orpolyethylene, but there are various other lightweight materials that canbe used. Particle size for the buoyant media may be in the range of 10to 500 microns, preferably in the range of 30 to 250 microns. The mediawill typically be added at a rate of 0.1 to 5 grams per liter of fluidmixture.

The fluid mixture, containing coagulants, flocculants, buoyant media,and the solids to be separated, then passes through a flotation chamberfeed line 24, into a flotation chamber 30. Flotation chamber 30 isconfigured so that entering fluid is directed upward, expediting theflotation process. A baffle 32 is an example of a structure directingentering fluid upward. The action of the buoyant media, in conjunctionwith the coagulant and flocculant, results in the formation of a thicksludge layer 34 at the top of flotation chamber 30. The lower region ofthe flotation chamber contains no outlet or mechanism for the removal ofsolids; all solids removed from flotation chamber 30 are removed fromthe upper region of flotation chamber 30. The sludge layer 34 isseparated from the flotation chamber 30 into a sludge hopper 36. Theseparation can be effected by a chain-and-flight or reciprocatingskimmer or other sludge removal device. Sludge can also be removed fromthe chamber 30 by hydraulic desludging. The removal of sludge,coagulant, and flocculant is effected from the upper region of flotationchamber 30. A clarified fluid 38 is formed by the removal of coagulantand flocculant from the entering fluid by the buoyant media. Optionally,a flotation assistance device 40 can be used to further hasten the flowof separated particles and buoyant media upward. Flotation assistancedevice 40 may be a dissolved air flotation (DAF) system or DAF pump, ordispersed air system or blower. The output from flotation assistancedevice 40 enters flotation chamber 30 through a flotation assistanceline 42.

Clarified fluid 38 is withdrawn from flotation chamber 30 through aclarified fluid withdrawal line 44 or other process outlet. Lamellaplates 45, tubes 45′ or an immersed membrane module 45″ can be added tothe flotation cell to hasten the separation process as shown in FIG. 2,FIG. 3 and FIG. 4. Optionally, a portion of the clarified fluid 38 canbe recycled through a clarified fluid recycling line 46 to flotationassistance device 40.

The sludge layer 34 is removed from sludge tank 36 through a sludge line48 to a buoyant media recovery unit 50. Buoyant media recovery unit 50may make use of a screening device to recover the buoyant flotationmedia, such as a vibratory separator, centrifugal screener, or across-flow sieve. The screens used are sized to selectively remove thebuoyant flotation particles. Other separation devices, such ascentrifuges, hydrocyclones, or fluidized bed classifiers, may also beused. The separated sludge is removed from buoyant media recovery unit50 through a separated sludge line 52. The separated buoyant flotationparticles are removed from buoyant media recovery unit 50 through abuoyant flotation particle recycling line 54 and recycled intoflocculation chamber 20.

Having described the currently preferred embodiment of the presentinvention, it is to be understood that the invention may be otherwiseembodied within the scope of the appended claims. For example, themodular tanks 10, 20 and 30 in FIG. 1 with pipe connections 14, 24 maybe replaced by a single large tank (not shown) having appropriateover-weir and under-weir devices to define separate chambers which arefluidly connected in the same general manner as shown in FIG. 1.

1. A process for separating suspended material from a fluid in a systemcomprising a flocculation chamber, a flotation chamber, a buoyant mediarecovery unit and a recycling line, said process comprising: (a) addingat least one coagulant and/or at least one flocculant to said fluid insaid flocculant chamber to form a mixed fluid; (b) adding buoyant mediato the mixed fluid; (c) directing the mixed fluid to said flotationchamber and separating the suspended material and buoyant media from themixed fluid by flotation resulting in floating sludge being formed in anupper portion of the flotation chamber and clarified fluid formed in alower portion of the flotation chamber; (d) removing the floating sludgefrom the flotation chamber; (e) removing clarified fluid from the lowerportion of the flotation chamber; (f) removing separated sludge from thesolid buoyant media in said buoyant media recovery unit; and (g)recycling at least a portion of said solid buoyant media afterseparation in step (f), via said recycling line, to said flocculationchamber.
 2. The process of claim 1 in which a portion of the clarifiedfluid is recycled to the flotation chamber.
 3. The process of claim 1 inwhich the buoyant media are finely divided solids with a specificgravity less than 1.0.
 4. The process of claim 1 in which the buoyantmedia are solids provided with internal voids.
 5. The process of claim 1additionally comprising introducing a gas to the mixed fluid in a lowerportion of the flotation chamber in step c).
 6. The process of claim 5wherein the gas is dissolved air.
 7. The process of claim 1 including instep c) the further step of passing the mixed fluid along an upwardlydirected baffle in the flotation chamber.